The invention relates a television transmission system in which the video components of a transmitted television signal are scrambled to allow only for their authorized reception, a video scrambling key being provided for the line-by-line scrambling or de-scrambling of said video components, said video scrambling key comprising a first pulse signal having a first pseudo random configuration generated at a first rate which corresponds to that of the line frequency, the first pulse signal being periodically set, in response to control data having a rate which is low compared with that of the line frequency, to a particular pulse configuration, which first pulse signal thereafter is stepped in accordance with its random configuration. The invention also relates to transmission and receiving equipment for use with such a system.
A system of the above type is known where each line of the video components of a television signal is scrambled, by cutting the line and changing the cut-point for each line. This is done in a random manner by a locally generated random video scrambling key at the transmission source and a corresponding random video scrambling key locally generated in the television receiver. Both scrambling keys are periodically reset by control data at a rate which is low relative to that of the line rate of the television signal, which control data is transmitted with the television signal. Such a method provides a relatively secure scrambling system for the video components.
With the above known system, no provision is made for scrambling or descrambling either sound or data signals transmitted with the video components of the television signal and there are circumstances where such scrambling could be required to prevent their unauthorized reception.